Many types of air separation plants are known from the prior art for separating air into its various components, chiefly oxygen and nitrogen. The processes used heretofore in such plants have been highly energy intensive, making the cost of the separated gases equally expensive. One reason for the high cost has been the fact that usually only high purity oxygen or nitrogen is produced, whether the high purity was needed or not. In circumstances where high purity was not needed, often the highly pure oxygen was simply diluted with air to the required purity or oxygen content.
Cryogenic air separation processes utilize the difference in boiling points of the various air components, principally oxygen and nitrogen, and the changes in boiling points with pressure to achieve component separation by fractional distillation. The basic process utilizes a number of auxiliary components such as compressors, expanders, heat exchangers, adsorbers, switching valves, and the like, whose function is to change the state of the input air stream so that component separation can take place and to condition the output streams to user requirements.
The actual separation (or rectification) process is usually carried out in "double rectification columns" which have an upper compartment operated at low pressure (near atmospheric in most cases) and a lower compartment operated at higher pressure. The lower compartment is usually referred to as the medium pressure column and the upper compartment, as the low pressure column. Both columns are provided with a number of perforated separation trays which in use bring about a series of fractional evaporations and condensations between a rising vapor which is successively enriched in nitrogen and a falling liquid which is being enriched in oxygen.
The process requirements for this heat and mass transfer to occur in the two phase, two component, countercurrent flow environment requires pressure differences between the two columns and thereby dictates the process energy input required for the air separation unit (ASU) air compressor. This is so since nitrogen, which at atmospheric pressure has a lower boiling point than the desired product oxygen, must condense at the top of the medium pressure column in order to evaporate product oxygen at the bottom of the low pressure column. The only way in which this condition may be met is for the condensation of nitrogen to take place at a higher pressure than that of the evaporating product oxygen.
It should be noted that the higher the desired oxygen concentration or purity of the product collecting at the bottom of the low pressure column, the greater the pressure difference requirement between the two columns and the higher the compressor power input for the same input air flow.
In many plants, the removal of water and carbon dioxide is combined with the cooling of the air, and this is done in reversing heat exchangers or regenerators.
In a typical prior art separation process, filtered atmospheric air is compressed in an intercooled and aftercooled air compressor to a pressure which is determined by the requirements of the particular process, usually about 4 to 6 atmospheres. It is then passed through a water separator and adsorber to heat exchangers and cooled to about 100.degree. K. (-280.degree. F.) with the aid of an expansion turbine.
The cold air is then injected as a saturated vapor into the medium pressure column where it is separated into a nitrogen fraction and a so-called rich liquid fraction containing about 40% oxygen. The separation is obtained by contacting the rising vapor with liquid nitrogen flowing down from the condensor-evaporator. This causes the liquid collecting at the bottom of the column to become enriched in oxygen while the vapor which is condensed at the top of the column to become enriched in nitrogen. Both fractions leave the lower column in the liquid state and flow through expansion valves to the low-pressure column where additional rectification to the final product purity takes place. Both nitrogen and oxygen leave the low pressure column in the gaseous state at near atmospheric pressure and are regeneratively heated by the process input air stream before being delivered to the user.
Traditionally, air separation plants have been designed to deliver high purity products (95 to 99.9 mol percent oxygen), even if the final application required a much lower oxygen purity. For example, in applications such as MHD plants or blast furnaces requiring only up to 40 percent oxygen concentration, but at elevated pressures, the product stream is mixed with atmospheric air before compression in specially designed uncooled oxygen-enriched air compressors. Alternatively, the product oxygen and the dilution air may be compressed to final user pressure in separate oxygen compressors and air compressors before being mixed to the desired oxygen concentration.
High purity air separation processes such as just described are not considered to be energy efficient. For example, such processes would require a specific energy consumption (SEC) in the range of 280 to over 300 kWh/(ton of equivalent pure oxygen), or 280 to 300 kWh/TEPO.
More recently, an advanced medium purity process has been developed in Europe which also requires external compression of the product stream, but which has reduced the energy consumption to approximately 224 kWh/TEPO by delivering a product containing only 60 mol percent oxygen. While this represents a significantly more energy efficient process, and is less hazardous than external compression of a high purity oxygen stream, the process still requires special turbomachinery having lower efficiencies than an equivalent air compressor because of the greater blade clearance and buffer gas bleed flow requirement. These safety precautions are required because the medium being compressed, although not as hazardous as pure oxygen, does have a higher oxygen content than normal air, and accordingly any ingested foreign particles such as dust, lubricating oil droplets, or abraded blade material can ignite more easily than in air.
Accordingly, a primary object of the present invention is to provide a safe, energy efficient process for separating gaseous mixtures.
A further object of the present invention is to provide a process for the separation of gaseous mixtures which is particularly useful for separating the component gases in air.
Still another object of the invention is to provide an energy efficient process for separating mixtures of gases of differing boiling points.
Another object of the invention is to provide a process for producing medium purity oxygen from air.
A further object of the invention is to provide an improved process for producing medium purity oxygen with low power consumption.
Still another object of the invention is to provide an improved, energy efficient process for producing medium purity oxygen at the concentration and pressure required by the user.
Yet a further object of the invention is to provide an energy efficient process capable of separating gaseous mixtures for producing a high purity gaseous product, such as oxygen, at significantly lower energy consumption than possible with prior art processes.